Sacrificial cathodic protection anodes, typically composed of alloys of magnesium, zinc, and aluminum, are usually connected directly to the structure being protected through a shielded cable or wire. Underground steel pipes are typical of the structures protected against corrosion in this way. Typically, the sacrificial anodic material encases a metallic core of lower galvanic activity than the anodic material, and connection to the anodic material is made through the metallic core. Commonly, the core is of steel, but in some cases copper or other metals can be utilized.
As the cathodic protection current is provided by the sacrificial anode material, the material of the anode is consumed until only the metallic core remains. At this time, the core is a liability to the structure, since it effectively adds additional area which must subsequently be unnecessarily protected. This is particulary important with steel cores, which are used in the majority of cases.
The steel cores left over from expended sacrificial anodes can represent an increasingly high percentage of the total bare area on a coated structure. For this reason, subsequent cathodic protection systems must be increased in capacity to supply current to the bare steel cores. This results in added protection costs in proportion to the amount of core area relative to the structure area. The only present means of avoiding this additional cathodic protection cost is to excavate and disconnect the core wire. However, this is a very costly procedure.